An artery is one of the tube-shaped blood vessels that carry blood away from a heart to the body's tissues and organs. An artery is made up of an outer fibrous layer, a smooth muscle layer, a connecting tissue layer, and the inner lining cells. If arterial walls become hardened due to the accumulation of fatty substances, then blood flow can be diminished. Hardening of the arteries, or loss of vessel elasticity, is termed arteriosclerosis while fatty deposit build-up is termed atherosclerosis. Atherosclerosis and its complications are a major cause of death in the United States. Heart and brain diseases are often the direct result of this accumulation of fatty substances that impair the arteries' ability to nourish vital body organs.
Recently, a new technique of inserting a metallic stenting element is used to permanently maintain the walls of the vessel treated at its extended opening state. Vascular stents are tiny mesh tubes or coils made of stainless steel or other metals and are used by heart surgeons to prop open the weak inner walls of diseased arteries. A catheter with a flexible guidewire-type tip is threaded up from the arm or groin through the artery until it reaches the blocked area. The stent is then deployed via an inflated balloon or via a delivery catheter. The deployed stent ruptures the plaque and increases the diameter of the blood vessel opening. The arterial passage is thus widened. As a result of enlarging the hardened plaque, cracks may unfortunately occur within the plaque to expose the underlying fresh tissue or denuded cells to the blood stream.
There are limitations, however, to this technique's application, depending on the extent of the disease, the blood flow through the artery, the part of the anatomy, and the particular vessels involved. Plaque build-up and/or severe re-stenosis recurrence within 6 months is generally up to 20-30 percent of those treated. The underlying newly exposed fresh collagen tissue or damaged cells still pose as a precursor for vessel reclosures or restenosis, regardless of stenting or not.
When a clogged artery is widened, the plaque or atheromatous material is broken up or split open while stretching the remaining soft parts of the vascular and perivascular tissue. Thus, stenting achieves its goal by creating a controlled but substantial injury to the vessel wall. However, the underlying collagen, tissue or damaged endothelium is exposed to the blood flow. Collagen has a pro-thrombotic property, which is part of the body healing processes. Furthermore, collagen has been widely used in hemostat treatment owing to its clotting properties. Unless the newly exposed collagen or the damaged endothelium is passivated or modulated, the chance for blood vessel clotting as well as restenosis still exists. Moderate heat is known to tighten and shrink the collagen tissue as illustrated in U.S. Pat. Nos. 5,456,662 and 5,546,954. It is also clinically verified that thermal energy is capable of denaturing the tissue and modulating the collagenous molecules in such a way that treated tissue becomes more resilient ("The Next Wave in Minimally Invasive Surgery" MD&DI pp. 36-44, August 1998). Therefore, it becomes imperative to post-treat vessel walls after the walls have been treated with angioplasty and/or stenting procedures.
One method of reducing the size of cellular tissues in situ has been used in the treatment of many diseases, or as an adjunct to surgical removal procedures. This method applies appropriate heat to the tissues, and causes them to shrink and tighten. It can be performed on a minimal invasive fashion, which is often less traumatic than surgical procedures and may be the only alternative method, wherein other procedures are unsafe or ineffective. Ablative treatment devices have an advantage because of the use of a therapeutic energy that is rapidly dissipated and reduced to a non-destructive level by conduction and convection, to other natural processes.
RF therapeutic protocol has been proven to be highly effective when used by electrophysiologists for the treatment of tachycardia; by neurosurgeons for the treatment of Parkinson's disease; and by neurosurgeons and anesthetists for other RF procedures such as Gasserian ganglionectomy for trigeminal neuralgia and percutaneous cervical cordotomy for intractable pains. Radiofrequency treatment, which exposes a patient to minimal side effects and risks, is generally performed after first locating the tissue sites for treatment. Radiofrequency energy, when coupled with a temperature control mechanism, can be supplied precisely to the device-to-tissue contact site to obtain the desired temperature for treating a tissue.
To effect the optimal ablation, it requires selection of the most appropriate device-to-tissue contact site as well as the most effective contact surface area. Several recent patents disclose a catheter in a basket structure having means for providing a plurality of discrete and isolated point electrodes. The patents include U.S. Pat. No. 4,699,147 to Chilson et al., U.S. Pat. No. 5,156,151 to Imran, U.S. Pat. No. 5,255,679 to Imran, U.S. Pat. No. 5,345,936 to Pomeranz et al., U.S. Pat. No. 5,411,025 to Webster, Jr., U.S. Pat. No. 5,628,313 to Webster, Jr., U.S. Pat. No. 5,636,634 to Kordis et al., and U.S. Pat. No. 5,672,153 to Lax et al. However, all of the above-identified patents comprise a non-conductive spacing between any two electrodes. A major drawback of those patents is obvious because of its limited electrode contact surface to the tissues for delivering heat therapy.
McGee et al. in U.S. Pat. No. 5,722,403 discloses a combination of a balloon and electrode arrangement for treating tissue. However, McGee et al. does not disclose a medical device comprising a plurality of continuous wire electrodes for contacting an implanted stent to treat the underlying exposed tissue for minimizing intravascular restenosis.
A stent deployed within a vessel, such as a coronary stent, has excellent metal-to-tissue contact surface. It becomes an ideal medium for applying thermal energy to the specific tissue that has been enlarged and has newly tissue exposed. The metal-to-tissue contact site is the tissue region that most urgently needs heat treatment or modulation. A RF delivery means for contacting the metallic stenting element is useful in this case to shrink and tighten the target tissue for treating intravascular restenosis. Particularly, a wire assembly arrangement comprising a plurality of deployable metallic members, such as the long continuous wires on a basket-type catheter shaft, is useful for delivering the RF thermal energy to the denuded collagen or damaged endothelium via a pre-implanted stent to shrink and tighten the target tissue after a stent-assisted angioplasty procedure.
Therefore, there is a need for an improved medical device having the capability to effectively contact the inner walls of a tubular vessel via a pre-implanted stent using the radiofrequency energy to treat an enlarged artery or other tissues, such as esophagus, larynx, ureter, urethra and the like.